Among the many photographs of the great dancer Vaslav Nijinsky is one that shows him, literally, in another light – backstage in the grey limbo of the wings. His head is barely upright. His eyes are almost closed, as if dazed with exhaustion, reverie or perhaps even drugs. He looks about to pass out.

The photograph was taken – only just – by the German-born, London-based photographer EO Hoppé. Nijinsky had posed for him several times before, and more conventional images exist of the Ballets Russes star displaying his extraordinarily athletic yet androgynous body. But Hoppé must have hoped for something more, something deeper – abandoning the studio to wait with his camera, backstage, while Nijinsky was performing the title role in Le Spectre de la Rose. Which is how the photograph came to be skimmed from nowhere: Nijinsky brushed past Hoppé, drained, at the end of the last show, but was persuaded to turn back for a second. Stillness and motion, standing or falling: this is the cusp.

Beyond the evident virtue of patiently watching and waiting, the image holds so many truths. Here is the isolation and singularity of the leading role, matched by the physical strain of its interpretation. Here, too, is Nijinsky's singularity, his curious beauty, his dark and knotted character, everything he has given to that night's performance – and to that part, for this photograph, more than any other, surely reveals the bizarre ambiguity of the Rose.

Intensely famous in his own time, almost forgotten in ours: the fate of so many painters has befallen this photographer. For Hoppé comes as a revelation. How can it be that an artist of such acute insights could have been so stinted that there have been only a handful of thematic shows (American and German portraits, images of Australia), and now the partial survey that opens at the National Portrait Gallery this week?

Perhaps the answer has something to do with his diversity. As Phillip Prodger argues in the show's excellent catalogue, Hoppé may have been the most socially mobile photographer in history, camping with Romanian gypsies, living with Australian aborigines, summoned by European monarchs, disappearing for months on end to record life in Cuba, Sri Lanka, the Bowery.

So many nations, so many sitters, from actors and aristocrats to butlers and bobbies. Hoppé may seem hard to pin down, but an artist's work is always about so much more than his range of subjects. There must be another reason why Hoppé is no longer a name.

Certainly it has nothing to do with the work. Cecil Beaton – who studied Hoppé from first to last – called him "the Master", and anyone might learn from his images. Indeed, learning seems central to his approach. There are no preconceptions, you feel, controlling Hoppé's portraits: each sitter is someone to discover from the start, someone to converse with, to try to comprehend.

Hoppé wanted the technicalities to be as simple as possible, he said, so he could focus on his rapport with the sitter. He didn't vanish under yards of black cloth but sat close with a cable release, so the shutter could be operated as unobtrusively as possible.

The face of the poet John Masefield, small, pale, enticed out of deep darkness, forms a frail triangle repeated by his collar. Both seem to be made out of paper. Jacob Epstein's hand and chisel merge into one organ as he stands beneath his tomb to Oscar Wilde – nervous, tense, defiant, perhaps already overshadowed by his own controversial creation.

Mussolini looks crazed, as if he might be about to snap. Perhaps it is in the nature of his face, but also in the faraway gaze that passes perilously close to Hoppé's lens, settling nowhere, taking in nothing of the occasion. It is 1924. What did Hoppé sense, or guess? So often his portraits appear prophetic.

Hoppé came to Britain to work for Deutsche Bank in 1902, at the age of 24. But he soon turned his self-taught hobby into a profession. His early photographs are painterly: the artist William Nicholson, gleaming head so far above his polished shoes he appears as elongated as a Sargent; Ballets Russes dancers, ephemeral as watercolours. But Hoppé learned something from everyone he portrayed.

Photographing actors, he looks to the hands when facial expression is too obviously controlled. Photographing poets, he narrows his depth of field, heightening crucial details, allowing backgrounds to melt to abstraction. Ezra Pound, in 1918, thrusts his chin abruptly at the lens, hair on end, collar raised like a devil's cloak, hieratic to the focused tip of his goatee.

As lights become more portable and cameras shrink in size, the world opens up for Hoppé. He can photograph the reclusive Thomas Hardy at home in Dorset in 1914. He can record the Chinese community in Limehouse, notoriously resistant to outsiders, by wrapping the quietest camera available (apparently a fixed-focus Brownie) in a paper bag with a slit for the lens, several years before Walker Evans took hidden shots on the New York subway.

The least interesting thing about Hoppé is, alas, probably the best known. In 1920 he travelled abroad in search of subjects for his The Book of Fair Women, a less pompous precursor to Lord Lichfield's Most Beautiful Women, which included, moreover, international sitters from all backgrounds. Such classification may be anathema now, but the book introduced a shattering new idea to the British press, namely that a Haitian country girl could outshine Lady Diana Duff-Cooper.

Hoppé was a close friend of George Bernard Shaw and an ardent progressive. If his politics are not immediately apparent in the celebrity portraits, his democratic compassion is visible everywhere. It is there in the portrait of a pearly king – or prince – aged about two, holding as still as he can in his button-stiff straitjacket, defending the little crust in his hand. It is there in the vivid hopefulness of Margot Fonteyn at 15, and the brisk Miss Vyse, taking a pride in her job as the original "Nippy" at Lyons' Corner House. It is in the grave dignity of the Indian flower-seller at Piccadilly Circus in 1921 and the woman nursing her Jack Russell at Croydon dog hospital.

What strikes, over and again, is the balance between dynamic clarity and softly benevolent light. The scene is scrutinised, the face is searched, then gently preserved. It feels like the graphic expression of Hoppé's own intellect: curiosity combined with empathetic insight.

What is it like to be the night watchman at the Bank of England, alone with your torch and top hat at the chimes of midnight (a melodrama not lost on the man himself, humorously colluding with Hoppé)? What does it feel like to be beautiful? One of Hoppé's greatest portraits shows the dancer Tilly Losch at point-blank range. Her luminous eyes are locked with yours, her startling face has its metaphor in the urgent directness of the close-up. Trapped between the lens and the wall, her beauty fills the frame: to her, as to us, inescapable.

Once seen, never forgotten; Losch lives in that image. But Hoppé himself begins to fade away. The last photographs in this exhibition show ways of life dying out even as his camera records them – a skeleton shop, a phrenologist still measuring customers' skulls as late as 1935. Hoppé was photographing Poland when war broke out. Forced home, he concentrated on establishing a library of images. He named it Dorien Leigh, not EO Hoppé, so his name was lost once – and then again, when he sold it to a much bigger library, where images were filed by subject and not by artist. A life's work, so many thousands of ideas, journeys, sitters and subjects diluted in a sea of other images: a reputation very nearly washed away.